Membrane made of a blend of UHMW polyolefins

ABSTRACT

A membrane is a microporous sheet made of a blend of a first ultra high molecular weight polyolefin and a second ultra high molecular weight polyolefin. Each polyolefin has a molecular weight, both of those molecular weights are greater than 1 million, and one molecular weight is greater than the other. Additionally, the intrinsic viscosity (IV) of the membrane may be greater than or equal to 6.3.

FIELD OF THE INVENTION

The present invention is directed to a microporous membrane made from ablend of ultra high molecular weight (UHMW) polyolefins.

BACKGROUND OF THE INVENTION

Microporous membranes are used in a variety of applications including:separators for electrochemical devices (e.g., batteries, fuel cells, andcapacitors), separation devices (e.g., mass transfer devices, andfiltration devices), medical devices (e.g., blood oxygenation anddialysis), pressure regulators, synthetic papers, to name a few.

The process for manufacturing microporous polyolefin membranes can bebroadly divided into the dry (or CELGARD) process and the wet process.

The CELGARD process involves melting a crystalline polyolefin resin,extruding the melt into a film, annealing the film, and orienting (orstretching) the film to form micropores. The CELGARD process involves noextraction step (i.e., solvent handling), and therefore it is inherentlysimpler than the wet process.

The wet process involves mixing of a polyolefin resin with a hydrocarbonliquid or some other low molecular weight substance, heating and meltingthe mixture, extruding the melt into a sheet, orienting (or stretching)the sheet, and extracting the liquid from the sheet with a volatilesolvent. The wet process is used to make microporous membranescontaining ultra high molecular weight polyethylene (UHMWPE).

The wet process, typically, includes one of the following phaseseparation mechanisms: (1) liquid-liquid phase separation; or (2)solid-liquid phase separation. The liquid-liquid phase separation, alsoknown as TIPS, refers to formation of a polymer-rich liquid matrix and adispersed polymer-lean liquid, with subsequent solidification of thepolymer. The solid-liquid phase separation, also known as GEL process,refers to polymer crystallization from a melt blend.

Microporous membranes made of UHMWPE which are filled or unfilled withparticulates are commercially available from Daramic of Owensboro, Ky.,Tonen Chemical of Tokyo, Japan, and Asahi Chemical of Tokyo, Japan.These types of membranes are also disclosed in, for example, U.S. Pat.Nos. 6,824,865; 6,666,969; 6,566,012; 6,245,272; 6,153,133; 6,096,213;5,993,954; 5,922,492; 5,853,633; 5,830,554; 5,786,396; 5,741,848;5,281,491; 5,051,183; 4,873,034; 4,734,196; 4,650,730; 4,620,955;4,600,633; 4,588,633; 4,539,256; and Japanese Patent Nos. 3497569 (KokaiJP08-064194); 3258737 (Kokai JP06-212006).

In one end use, battery separators for secondary lithium ion batteries,there is a continuing demand to decrease the thickness of the separatorwhile maintaining or increasing its strength (i.e., the puncturestrength).

Accordingly, there is a need for new membranes to meet these and otherrequirements.

SUMMARY OF THE INVENTION

A membrane is a microporous sheet made of a blend of a first ultra highmolecular weight polyethylene and a second ultra high molecular weightpolyethylene. Each polyethylene has a molecular weight, both of thosemolecular weights are greater than 1 million, and one molecular weightis greater than the other. Additionally, the intrinsic viscosity (IV) ofthe membrane is greater than or equal to 6.3.

DESCRIPTION OF THE DRAWINGS

For the purpose of illustrating the invention, there is shown in thedrawings a form that is presently preferred; it being understood,however, that this invention is not limited to the precise arrangementsand instrumentalities shown.

FIG. 1 is a graphical illustration of the relationship between thepercent of processing oil remaining in the precursor before stretchingand average pore size.

FIG. 2 is a graphical illustration of the relationship between theporosity of the membrane and the average pore size.

DESCRIPTION OF THE INVENTION

A membrane refers to a microporous sheet made from a blend of at leasttwo ultra high molecular weight polyolefins. These blends will bediscussed in greater detail below. The membrane or microporous sheet maybe characterized by one or more of the following parameters: thickness,porosity, average pore size, puncture strength, MacMullin Number, GurleyNumber, intrinsic viscosity (IV), and shutdown temperature. Each ofthese will be discussed below.

The thickness of the membrane may be less than 5 mils (125 microns). Inanother embodiment, the thickness may range from 10 microns to 50microns. In yet another embodiment, the thickness may range from 10microns to 25 microns.

The porosity of the membrane may be between 25 and 85%. In oneembodiment, porosity ranges from 32-58%. In yet another embodiment,porosity ranges from 43-57%.

The average pore size of the membrane may be between 0.01-0.5 microns.In one embodiment, the average pore size ranges from 0.025-0.09 microns.In yet another embodiment, the average pore size ranges from 0.027-0.054microns.

The puncture strength may be greater than or equal to 300 gr-force/mil.Puncture strength is determined by averaging 10 measurements across thewidth of the final product using a Midtech Stevens LFRA texture analyzerand a needle with a 1.65 mm diameter and a 0.5 mm radius recording dataat a rate of 2 mm/sec with a maximum amount of deflection of 6 mm.

The MacMullin Number (N_(mac)) may be in the range of 6-15. MacMullinNumber is a measure of resistance to movement of ions. MacMullin Numberis the ratio of the resistance (r) of the electrolyte saturatedseparator to the resistance (r₀) of an equivalent volume of electrolyte.(N_(mac)=r/r₀). Also see U.S. Pat. No. 4,464,238 which is incorporatedherein by reference.

The Gurley Number (normalized to one mil thickness) may be less than 60sec/10 cc/mil thickness. In one embodiment, the Gurley number rangesfrom 12 to 56 sec/10 cc/mil. The Gurley Number is the time in secondsrequired to pass 10 cc of air through one square inch of a film under apressure of 12.2 inches of water as measured by a Gurley densometer(e.g., Model 4120) according to ASTM D-726(B).

The intrinsic viscosity (IV) of the membrane may be greater than orequal to 6.3 dl/g. In another embodiment, the IV may be greater than orequal to 7.7 dl/g. The IV of the film is not the weighted average of thepre-extruded resins composing the membrane because during extrusion thepolymers undergo chain scission and the molecular weight is therebylowered. Intrinsic viscosity, as used herein, refers to the measure ofthe capability of a polymer in solution to enhance the viscosity of thesolution. The intrinsic viscosity number is defined as the limitingvalue of the specific viscosity/concentration ratio at zeroconcentration. Thus, it becomes necessary to find the viscosity atdifferent concentrations, and then extrapolate to zero concentration.The variation of the viscosity number with concentration depends on thetype of molecule as well as the solvent. In general, the intrinsicviscosity of linear macromolecular substances is related to the weightaverage molecular weight or degree of polymerization. With linearmacromolecules, viscosity number measurements can provide a method forthe rapid determination of molecular weight when the relationshipbetween viscosity and molecular weight has been established. IV ismeasured by first dissolving 0.02 g of the membrane in 100 ml of decalinat 150° C. for one hour, and then, determining its intrinsic viscosityat 135° C. via an Ubbelohd viscometer. This is according to ASTM D4020(RSV values reported herein).

The shutdown temperature may be less than 140° C. In one embodiment, theshutdown temperature may be less than 135° C. In yet another embodiment,the shutdown temperature may be less than 130° C.

The membrane may be a blend of ultra high molecular weight polyolefinshaving differing molecular weights. In one embodiment, these ultra highmolecular weight polyolefins may be ultra high molecular weightpolyethylene (UHMWPE). In another embodiment, the membrane is a blend ofa first ultra high molecular weight polyethylene having a firstmolecular weight and a second ultra high molecular weight polyethylenehaving a second molecular weight, the first molecular weight and thesecond molecular weight being greater than 1 million and being differentfrom one another. In another embodiment, the membrane is a blend of afirst ultra high molecular weight polyethylene having a first molecularweight, a second ultra high molecular weight polyethylene having asecond molecular weight, the first molecular weight and the secondmolecular weight being greater than 1 million and being different fromone another, and a third polyolefin having a third molecular weight, thethird molecular weight being less than 1 million. In yet anotherembodiment, the membrane may have an IV greater than or equal to 6.3dl/g. In another embodiment, the membrane may have an IV greater than orequal to 7.7 dl/g.

The first ultra high molecular weight polyethylene may have an IV in therange of 7<IV<15. For example, such materials are commercially availablefrom Ticona of Florence, Ky. as GUR 4012 (IV=10).

The second ultra high molecular weight polyethylene may have an IV ofgreater than or equal to 15. For example, such materials arecommercially available from: Ticona of Florence, Ky. as GUR 4120 (4022)(IV=21) and 4130 (4032) (IV=24); and DSM of Beek, Netherlands asSTAMYLAN UH 034 (IV=15).

The third polyolefin may be any ‘melt processable’ polyolefin. ‘Meltprocessable,’ as used herein, means that the polymer will flow whenmelted at its melt temperature. Such melt processable polymers may havea molecular weight less than 1.0×10⁶ and may have a molecular weightless than or equal to 0.5×10⁶. Such polymers include, for example,polyethylene, polypropylene, polybutylene, polypentene. In oneembodiment, the polyolefin is polyethylene, for example high densitypolyethylene (HDPE). For example, such HDPE's are commercially availablefrom Ticona of Florence, Ky. as GHR 8020 (MW=0.35×10⁶ and 2110(MW=0.5×10⁶). One function of this component is to allow shutdown whenthe membrane is used as a battery separator.

The membrane IV values are measured after formation of the microporoussheet. The membrane IV value is not merely the weighted average of theIV's of the starting materials because during mixing and gellation inthe extruder, the polymers undergo chain scission. Chain scission thusresults in an IV lower than the weighted average of the startingcomponents.

In those embodiments of the membrane with only the first and secondUHMWPE components, the blends may comprise any weight percent of bothcomponents, for example 1-99% of each component.

In one embodiment, the blend may comprise 20-80% by weight of the resinsof the first component and 80-20% by weight of the second component. Inanother embodiment with only the first and second components, the blendmay comprise 40-60% of the first component and 60-40% of the secondcomponent.

In those embodiments of the membrane with the first UHMWPE, secondUHMWPE, and third polyolefin, the blend may comprise a weight ratio offirst:second:third components of 1-6:1-6:1-6. In another embodiment ofthis membrane, the ratio of components may be in the range of1-6:1-3:1-6. In yet another embodiment, the blend may comprise a weightratio of 1-5.5:1-2:1-5.5.

The membrane may be manufactured by a wet process. The initial resincomponents may be formed into a dry blend by mixing the initial resincomponents with about 10% of a processing oil, discussed below. This dryblend may be charged into a twin screw extruder for mixing with thebalance of the processing oil being injected into the extruder prior tokneading of all components in the extruder. This mixture may be extrudedinto a nonporous sheet through a slot die. Optionally, any portion, orall, of the processing oil may be removed from the sheet, via extraction(conventional), prior to orientation (stretching). The sheet may besubjected to orientation, for example, biaxial orientation. Thereafter,if necessary, any remaining processing oil is extracted (i.e.,conventional extraction techniques). Alternatively, the initial polymercomponents may be charged directly into the extruder and all of the oilinjected into the extruder prior to the kneading section(s) of theextruder, and then processed as set out above. In either embodimentdescribed above, it is also possible to inject a portion of the oilprior to the first kneading section in the extruder and then inject thebalance of the oil after the first kneading section(s). While notnecessary, the inclusion of VEL (viscoelastic lubricant), discussedbelow, has been found beneficial to the processing of the forgoing. Atextrusion, the resin may comprise about 10-55% by weight of the totalmixture, the balance being processing oil or processing oil and VEL. Inanother embodiment, at extrusion, the resin may comprise about 10-40% ofthe total mixture.

Processing oil has little solvating effect on the UHMWPE at lowertemperatures (e.g., 60° C.), but has a significant solvating effect atelevated temperatures (e.g., 200° C.). Such oils include paraffinicoils, naphthenic oils, and aromatic oils, as well as other materialsincluding the phthalate ester plasticizers such as dibutyl phthalate,bis(2-ethylene)phthalate, diisodecyl phthalate, dicyclohexyl phthalate,butyl benzyl phthalate, and ditridecyl phthalate. Additional oils,plasticizers, and/or solvents are mentioned in U.S. Pat. Nos. 3,351,495;4,588,633; 4,833,172; 5,248,461; and 5,948,557, and only the relatedportions of the foregoing patents with regard to such oils,plasticizers, and/or solvents are incorporated herein by reference.

VEL are compounds that, when added to the foregoing resin mixtures,improve the processability of the mixtures. Improved processabilityrefers to a reduction in fusion time (the time it takes the polymericsystem to melt (or dissolve) into a flowable solution). Improvedprocessability is also seen as a reduction in energy consumption by theextruder motor and as a reduction in mixture temperature when comparingsystems with and without the lubricants. The results arising from thisphenomenon include, but are not limited to, decreasing energyconsumption, decreased thermal and mechanical degradation of thepolymer, increased polymer strength, decreased machine wear, andincreased polymer throughputs.

Such VELs are selected from the material classes consisting of: fattyacid esters, ethoxylated fatty acid esters, glycol esters, PEG esters,glycerol esters, ethoxylated esters, sorbitol esters, ethoxylatedsorbitol esters, aromatic ethoxylates, alcohol ethoxylates, mercaptanethoxylates, modified ethoxylates, amide surfactants, phosphate esters,phosphonate esters, phosphite esters, alkyl sulfates, fatty acid ethers,alkyl ether sulfates, alkylaryl ether sulfates, sulfonates, naphthalenesulfonates, sulfosuccinates, sulfonated esters, sulfonated amides, alkylether carboxylates, alkylaryl ether carboxylates, quaternary amines,amino quaternary amines, ethoxylated amines, imidazoline derivatives,betaines, sultaines, aminopropionate, catechol derivatives, saturatedfatty acids, unsaturated fatty acids, and combinations thereof. Forexample, such lubricants are commercially available. An exemplary listis set out in the Table below.

TABLE Tradename or General Class of Abbreviation Surfactants Specificchemical Company Rhodasurf ® LA-12 Alcohol Ethoxylates Mixed linearalcohol Rhodia HPCII ethoxylate Rhodasurf ® LA-3 Alcohol EthoxylatesMixed linear alcohol Rhodia HPCII ethoxylate Rhodapex ® CD-128 Alkyl(and Alkyllaryl) Ammonium Linear Rhodia HPCII Ether Sulfate AlcoholEther Sulfate Rhodapon ® BOS Alkyl Sulfates Sodium 2-ethylhexyl RhodiaHPCII Sulfate Rhodapon ® UB Alkyl Sulfates Sodium Lauryl Rhodia HPCIISulfate Alkamide ® STEDA/B Amide surfactant Ethylene Rhodia HPCIIBisstearamide Igepal ® CO-210 Aromatic Ethoxylates Nonylphenol RhodiaHPCII ethoxylates Igepal ® CO-630 Aromatic Ethoxylates NonylphenolRhodia HPCII ethoxylates Igepal ® RC-630 Aromatic Ethoxylates DodecylPhenol Rhodia HPCII Ethoxylates Mirataine ® CBS Betaines, Sultaines,Coco/Oleamidopropyl Rhodia HPCII and Aminopropionates BetaineMirataine ® COB Betaines, Sultaines, Cocamidopropyl Rhodia HPCII andAminopropionates Hydroxy Sultaine Miranate ® LEC-80 Ether CarboxylateSodium Laureth 13 Rhodia HPCII Carboxylate Rhodameen ® PN-430Ethoxylated Fatty Ethoxylated (5 Rhodia HPCII Amines moles) tallow amineRhodameen ® T-50 Ethoxylated Fatty Ethoxylated (50 Rhodia HPCII Aminesmoles) tallow amine Calcium Stearate Fatty acids, saturated Calciumstearate Linseed Oil Fatty acids, Linoleic and Hardware unsaturatedlinolenic acids Store Tung Oil Fatty acids, Eleostearic acid Hardwareunsaturated Store Alkamuls ® GMS Glycerol ester Glycerol stearate RhodiaHPCII Kemester ® 1000 Glycerol trioleate Glycerol trioleate CromptonCorp. Alkamuls ® EGDS Glycol ester Glycol distearate Rhodia HPCIIAlkamuls ® JK Guerbet ester Guerbet diester Rhodia HPCII Neustrene ® 059Hydrogenated tallow (30% Palmitic, 60% Crompton glycerol Stearic) Corp.Neustrene ® 064 Hydrogenated tallow (88% Stearic, 10% Crompton glycerolPalmitic) Corp. Miranol ® C2M-SF Imidazoline derivative DisodiumCocoampho Rhodia HPCII Dipropionate Miranol ® JEM Imidazoline derivativeSodium Mixed C8 Rhodia HPCII Amphocarboxylate Antarox ® 724/P EthoxylateRhodia HPCII Rhodacal ® N Naphthalene Sodium Naphthalene Rhodia HPCIIFormaldehyde Formaldehyde Sulfonates Sulfonate Supragil ™ WP NaphthaleneSulfonates Sodium Diisopropyl Rhodia HPCII Naphthalene SulfonateAlkamuls ® EL-620 PEG Ester PEG-30 Castor Oil Rhodia HPCII (ricinoleic +oleic + palmitic . . . ) Duraphos ® 2EHA Phosphate Ester PhosphoricAcid, Rhodia HPCII PO4 Mono & Di(2- ethylhexyl) ester DEHPA ® extractantPhosphate Ester Phosphoric Acid, Rhodia HPCII Bis(2- ethylhexyl) esterRhodafac ® LO-11A Phosphate Ester Phosphoric Acid, Rhodia HPCII LA Blendof linear octyl/decyl alcohol esters Amgard ® TOF Phosphate EsterPhosphoric Acid, Rhodia HPCII Tris(2- ethylhexyl) ester Albrite ® B(2EH)Phosphonate Ester Phosphonic Acid, (2- Rhodia HPCII 2EHPethylhexyl)-bis(2- ethylhexyl) ester Octylphosphonic Phosphonate EsterOctyl Phosphonic Rhodia HPCII Acid Acid Ester Rhodaquat ® QuaternaryAmine Complex ditallow Rhodia HPCII DAET-90 sulfate quaternary amineAlkamuls ® SML Sorbitan ester Sorbitan Monolaurate Rhodia HPCIIAlkamuls ® SMO Sorbitan ester Sorbitan Monooleate Rhodia HPCIIAlkamuls ® STO Sorbitan ester, Sorbitan Trioleate Rhodia HPCIIethoxylated OT-75, OT-100 Sulfosuccinates Dioctyl sodium Cytecsulfosuccinate

Another aspect of the invention relates to a method of controlling thephysical properties of the membrane by pre-extraction of a portion ofthe processing oil (or processing oil and VEL) prior to orientation. Ithas been determined that physical properties may be strongly affected bythe processing oil content of the precursor during orientation. Thus,one may control the physical properties of the membrane by the amount ofoil removed from the membrane before orientation.

For example, average pore size and porosity are strongly affected by theoil content of the precursor prior to orientation. Referring to FIG. 1,a relationship is demonstrated between the % oil (or oil and VEL)remaining in the precursor during stretching (x-axis) and the averagepore size (y-axis). Curve 1 illustrates the relationship when theinitial (extruded) mixture comprises about 24% resin in oil. Curve 2illustrates the relationship when the initial (extruded) mixturecomprises about 30% resin in oil. One will appreciate that by extractingoil before orientation, then average pore size may be increased.Referring to FIG. 2, a relationship between porosity of the membrane andthe average pore size (as determined by the % oil (or oil and VEL)remaining in the precursor during stretching, as demonstrated in FIG. 1)is illustrated. In FIG. 2, the porosity was determined from the membranesamples illustrated in FIG. 1. One will appreciate that by extractingoil before stretching, one will increase the porosity.

EXAMPLES

The present invention is further illustrated by the followingnon-limiting examples. The property values set forth in the Tables are acompilation of individual runs at the stated conditions. In all of thefollowing examples the initial solution of processing oil (or processingoil and VEL, as indicated) and resins were extruded, via a twin screwextruder, through a slot die, then either 1) subjected to extraction(partial or complete) of the processing oil, then stretched (biaxially),and finally, if necessary, full extraction of any remaining processingoil, or 2) subjected to no pre-stretching extraction, then stretched(biaxially), and finally full extraction of the processing oil.Experimental work included formulations with and without viscoelasticlubricants (VEL). Percent composition means % by weight of the totalblend. Ratios are expressed as weight ratios.

In an initial experiment, a microporous film was produced from aformulation containing 33% of HDPE, 33% of a first UHMWPE (IV=10), and33% of a second UHMWPE (IV=24) with an oil to resin ratio of 3:1, but noVEL. From this work, it was determined that during extrusion, the resinmixture loses between 20-30% of its IV when compared to the weightedaverage IV of the pre-extruded resin blend. Thus, while it was possibleto make the microporous membranes without VEL, the use of VEL ispreferred.

In subsequent experimentation, microporous films were produced fromformulations set out in Table 1.

TABLE 1 UHMWPE UHMWPE UHMWPE UHMWPE Formulation Oil:VEL HDPE IV = 10 IV= 15 IV = 21 IV = 24 A 10:1  — 50 — — 50 B 3:1 33 33 — 33 — C 3:1 33 33— 33 — D 3:1 33 33 — — 33 E 3:1 40 20 — — 40 F 3:1 33 33 33 — — G 3:1 5510 — — 35The processing parameters and film properties are set out in Tables 2(formulations with blends of a first UHMWPE and a second UHMWPE) and 3(formulations with blends of HDPE, first UHMWPE, and second UHMWPE).

TABLE 2 % Polymer Stretch Stretch Puncture Polymer at Temp RatioThickness Gurley Strength Pore Size (X) Extrusion C. (Y) microns Sec/milgr-force Porosity microns A 24 122 4 × 4  24-28, 36-38 416 38 0.029 A 24122 5 × 5 16-19 36-42 406-437 40-43 0.024-0.028 A 24 120 4 × 4 13-23 9-31 184-271 48-51 0.037-0.049 A 24 125 4 × 4 11-19 23-49 131-178 46-570.030-0.050 A 24 125 5 × 5 11-18 26-35 166-205 47-53 0.026-0.043 A 30122 4 × 4 14-22 26-41 333-395 40-45 0.037-0.037 A 30 125 4 × 4 12-2821-56 226 43 0.039 A 25 115, 120 3.6-4.4 12.4-19.1 24.3-35.5

TABLE 3 % Polymer Stretch Stretch Puncture Pore Polymer at Temp RatioFilm Thickness Gurley Strength Size Tortu- Shutdown (X) Extrusion C. (Y)IV microns Sec/mil gr-force MacMullin Porosity microns osity Temp C. B30 120 4 × 4 7.7 28  9-12 367 51 0.041 B 30 120 5 × 5 7.7 14-17 22-32321-340 10.9 46-48 0.027-0.041 2.3 B 30 122 4 × 4 7.7 21 10-11 254 6.155 0.047 1.8 B 30 122 5 × 5 7.7 12-17 19-27 256-342 9.5 50-530.040-0.055 2.2 B 30 122 6 × 6 7.7 14-18 27-33 383-393 9.1-11.9 47-490.029-0.032 2.1-2.4 B 30 125 4 × 4 7.7 19-20  7-23 213 49 0.041 B 30 1255 × 5 7.7 13-17 16-21 173-249 47-48 0.051-0.060 B 30 125 6 × 6 7.7 12-1313 219 54 0.044 C 25 115, 120 3.9-4.8 9.0 11.2-18.0 25.7-44.7 254-4077-15 47-50 0.033-0.038 1.9 146 D 25 115, 120 3.6-4.8 9.2 11.4-21.616.3-53.0 260-353 7-12 43 0.029 151 D 25 115, 120 3.5-4.6 9.5 11.7-20.814.8-54.9 313-322 8-12 49-54 0.031-0.041 2 E 25 115, 120 3.4-4.9 8.912.2-21.1 28.4-52.4 358-383 10 48 0.033-0.038 2.2 145 F 25 115, 1203.7-4.5 7.6 10.9-17.5 12.8-53.5 235-387 6-8  51-57 0.037-0.054 1.8 145 G25-35 115, 120 3.4-4.5 11.2-22.4 25.6-52.3 133

The present invention may be embodied in other forms without departingfrom the spirit and the essential attributes thereof, and, accordingly,reference should be made to the appended claims, rather than to theforegoing specification, as indicated the scope of the invention.

We claim:
 1. A membrane comprises: an extruded, wet processedmicroporous sheet consisting essentially of a first ultra high molecularweight polyethylene having a first molecular weight, as measured byintrinsic viscosity (IV), in a range of 7<IV<15 and a second ultra highmolecular weight polyethylene having a second molecular weight, asmeasured by intrinsic viscosity (IV), in a range of IV>15, said firstmolecular weight and said second molecular weight being greater than 1million and being different from one another, the microporous sheethaving a thickness in a range of 10-25 microns and interconnectedspherical and/or elliptical pores, and characteristics selected from thegroup consisting of: a porosity between 25 and 85%; a puncture strengthof greater than 300 gr-force/mil (25 microns); a MacMullin Numberbetween 6-15; a Gurley Number of less than 60 sec/10 cc/mil; andcombinations thereof.
 2. The membrane of claim 1 where said microporoussheet having a porosity ranging from 32 to 58%.
 3. The membrane of claim1 where said microporous sheet having an average pore size between 0.01to 0.5 microns.
 4. The membrane of claim 1 where said microporous sheetfurther consisting of a viscoelastic lubricant (VEL), said VEL beingselected from the group consisting of: fatty acid esters, ethoxylatedfatty acid esters, glycol esters, PEG esters, glycerol esters,ethoxylated esters, sorbitol esters, ethoxylated sorbitol esters,aromatic ethoxylates, alcohol ethoxylates, mercaptan ethoxylates,modified ethoxylates, amide surfactants, phosphate esters, phosphonateesters, phosphite esters, alkyl sulfates, fatty acid ethers, alkyl ethersulfates, alkylaryl ether sulfates, sulfonates, naphthalene sulfonates,sulfosuccinates, sulfonated esters, sulfonated amides, alkyl ethercarboxylates, alkylaryl ether carboxylates, quaternary amines, aminoquaternary amines, ethoxylated amines, imidazoline derivatives,betaines, sultaines, aminopropionate, catechol derivatives, saturatedfatty acids, unsaturated fatty acids, and combinations thereof.
 5. Themembrane of claim 1 wherein the microporous sheet further comprises athird polyolefin with a molecular weight of less than 0.5×10⁶.
 6. Themembrane of claim 1 wherein said sheet being made of a solutionincluding said first ultra high molecular weight polyethylene, saidsecond ultra high molecular weight polyethylene, and a hydrocarbonliquid or some other substance which is at least partially extracted bya volatile solvent.
 7. A battery separator comprising the membrane ofclaim
 1. 8. A membrane comprises an extruded, wet processed microporoussheet being made of a solution of a first ultra high molecular weightpolyethylene having a first molecular weight, as measured by intrinsicviscosity (IV), in a range of 7<IV<15 and a second ultra high molecularweight polyethylene having a second molecular weight, as measured byintrinsic viscosity (IV), in a range of IV>15, said first molecularweight and said second molecular weight being greater than 1 million andbeing different from one another, the microporous sheet having athickness in a range of 10-25 microns, an average pore size consistingof a range of 0.01-0.5 microns, and interconnected spherical and/orelliptical pores, and characteristics selected from the group consistingof: a porosity between 25 and 85%; a puncture strength of greater than300 gr-force/mil (25 microns); a MacMullin Number between 6-15; a GurleyNumber of less than 60 sec/10 cc/mil; and combinations thereof.
 9. Themembrane of claim 8 wherein the membrane having an intrinsic viscositygreater than or equal to 6.3.
 10. The membrane of claim 9 wherein saidintrinsic viscosity being greater than or equal to 7.7.
 11. The membraneof claim 8 wherein the microporous sheet further consisting of aviscoelastic lubricant (VEL), said VEL being selected from the groupconsisting of: fatty acid esters, ethoxylated fatty acid esters, glycolesters, PEG esters, glycerol esters, ethoxylated esters, sorbitolesters, ethoxylated sorbitol esters, aromatic ethoxylates, alcoholethoxylates, mercaptan ethoxylates, modified ethoxylates, amidesurfactants, phosphate esters, phosphonate esters, phosphite esters,alkyl sulfates, fatty acid ethers, alkyl ether sulfates, alkylaryl ethersulfates, sulfonates, naphthalene sulfonates, sulfosuccinates,sulfonated esters, sulfonated amides, alkyl ether carboxylates,alkylaryl ether carboxylates, quaternary amines, amino quaternaryamines, ethoxylated amines, imidazoline derivatives, betaines,sultaines, aminopropionate, catechol derivatives, saturated fatty acids,unsaturated fatty acids, and combinations thereof.
 12. The membrane ofclaim 8 wherein said solution also includes a hydrocarbon liquid or someother substance which is at least partially extracted by a volatilesolvent.
 13. The membrane of claim 8 wherein the microporous sheetfurther comprises a third polyolefin with a molecular weight of lessthan 0.5×10⁶.
 14. A battery separator comprising the membrane of claim8.
 15. A lithium battery comprising: a membrane of an extruded, wetprocessed microporous sheet being made of a solution of a first ultrahigh molecular weight polyethylene having a first molecular weight, asmeasured by intrinsic viscosity (IV), in a range of 7<IV<15 and a secondultra high molecular weight polyethylene having a second molecularweight, as measured by intrinsic viscosity (IV), in a range of IV>15,said first molecular weight and said second molecular weight beinggreater than 1 million and being different from one another, themicroporous sheet having a thickness in a range of 10-25 microns, anaverage pore size consisting of a range of 0.01-0.5 microns,interconnected spherical and/or elliptical pores, and characteristicsselected from the group consisting of: a porosity between 25 and 85%; apuncture strength of greater than 300 gr-force/mil (25 microns); aMacMullin Number between 6-15; a Gurley Number of less than 60 sec/10cc/mil; and combinations thereof.
 16. The membrane of claim 15 whereinthe microporous sheet further comprises a third polyolefin with amolecular weight of less than 0.5×10⁶.
 17. A lithium battery comprising:a membrane of an extruded, wet processed microporous sheet consistingessentially of a first ultra high molecular weight polyethylene having afirst molecular weight, as measured by intrinsic viscosity (IV), in arange of 7<IV<15 and a second ultra high molecular weight polyethylenehaving a second molecular weight, as measured by intrinsic viscosity(IV), in a range of IV>15, said first molecular weight and said secondmolecular weight being greater than 1 million and being different fromone another, the microporous sheet having a thickness in a range of10-25 microns, interconnected spherical and/or elliptical pores, andcharacteristics selected from the group consisting of: a porositybetween 25 and 85%; a puncture strength of greater than 300 gr-force/mil(25 microns); a MacMullin Number between 6-15; a Gurley Number of lessthan 60 sec/10 cc/mil; and combinations thereof.
 18. The membrane ofclaim 17 wherein the microporous sheet further comprises a thirdpolyolefin with a molecular weight of less than 0.5×10⁶.